Configure Microsoft Exchange to reject emails for invalid recipients

Root > Hornet Security

Rejecting invalid recipient for Exchange is needed for Hornet Security to know if the user exist or not and avoiding customer to exceed the license agreement.

Exchange 2003:

To enable Microsoft Exchange Server 2003 to allow emails for valid recipients only, please follow this steps. 
First, enable filtering for recipients which are not found in Active Directory.

  1. Open Exchange System Manager > Global Settings, right-click on Message Delivery, and choose Properties.
  2. Go to the Recipient Filtering tab.
  3. Enable the option Filter recipients who are not in the Directory.
  4. Click OK to close the window and save your changes.

Then, enable the recipient filter on the SMTP Virtual Server. This will only need to be enabled on the SMTP virtual server that is receiving emails from the internet.

  1. Open Exchange System Manager > Administrative Groups > (Administrative Group Name) > Servers > Protocols > SMTP.
  2. Right-click on the SMTP Virtual Server and select Properties.
  3. Go to the General tab click the Advanced... button.
  4. Choose the IP binding that that is listening on the Internet. Click the Edit... button.
  5. Enable the Apply Recipient Filter option.
  6. Click OK through all the windows to save your changes.
When someone tries to send an email to a user that does not exist in your Active Directory domain, they will receive this error: 


550 5.5.1 User unknown


Exchange 2007/2010

Recipient Verification is configured in the "Anti-Spam agents" module. The Anti-Spam agents are enabled by default on Edge Transport servers, but not Hub Transport servers. If you do not have an Edge Transport Exchange 2007 server, you can enable the Anti-Spam agents on a Hub Transport server. Please follow this link to a Microsoft knowledgebase article below if you need to enable the "Anti-Spam agents" module on your Exchange 2007 server.

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http://support.microsoft.com/kb/555924

Once you have verified that the Anti-Spam agents are enabled, you can configure Exchange to block mail addressed to recipients that don't exist at your organization. This feature is called Recipient Lookup, and can be enabled by following these steps:

  1. Open the Exchange Management Console.
  2. Expand Organization Configuration.
  3. Click on Hub Transport.
  4. Click the Anti-Spam Tab.
  5. Double click Recipient Filtering.
  6. Click the Blocked Recipients tab.
  7. Check the first option, labeled Block messages sent to recipients not listed in the Global Address list.
When someone tries to send an email to a user that does not exist in your Active Directory domain, they will receive this error: 


550 5.5.1 User unknown


Exchange 2013

Configuration for Exchange 2013 can be referred to here
Article ID: 140, , Modified: 10/11/2018 at 2:54 PM

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